The U.S. Pat. No. 4,322,379 discloses a technology for producing tear-drop shaped crystals of substantially uniform size, in which semiconductor silicon is heated inside the upper end portion of a drop tube made of quartz in order to obtain a melt, the gas pressure of helium gas is caused to act on this melt, and the particulate melt thus obtained is allowed to free-fall inside the drop tube and solidified during the free fall. However, the particulate melt does not go into microgravity because it receives the drop resistance of the gas inside the drop tube.
The inventor of the present application has disclosed a drop tube type spherical crystal producing device capable of producing particulate semiconductor single crystals, as indicated in the U.S. Pat. No. 6,204,545. In this spherical crystal producing device, a semiconductor particulate is turned into a melt in a floating state in the upper end portion of an approximately 14-meter-long drop tube, and this melt is allowed to free-fall inside the drop tube and is solidified in microgravity while falling, by means of radiation cooling, to obtain a spherical semiconductor single crystal. The drop tube of this spherical crystal producing device is configured to have the same diameter throughout the length thereof, but not to cool the melt by using cooling gas.
In such kind of spherical crystal producing device, a particulate melt is merely cooled by means of radiation cooling, which takes a long cooling time and requires a tall drop tube, thus equipment cost becomes expensive. Moreover, it is difficult to cool all over the melt evenly. In a case of a melt of silicon or the like, the melt has a property of expanding when being solidified, thus, cooling the whole melt unevenly distorts the shape of a solidified spherical crystal easily.
In the U.S. Pat. No. 6,106,614, a drop tower type spherical crystal producing device is proposed. In this device, a crucible made of quartz is provided on the upper end side of a drop tower, a powdery semiconductor (for example, semiconductor silicon) is supplied to the crucible from the outside, and at the same time the semiconductor is melted, and vibration is added to the melt inside the crucible by a vibration adding mechanism, whereby a particulate melt is allowed to fall inside the drop tower from a nozzle on the lower end of the crucible. In a nucleation zone at the mid section of the drop tower, there are provided cooling gas flow formation means for generating a flow of cooling inert gas going from bottom to top, and seed crystal generation means for generating a seed crystal in the falling particulate melt. The particulate melt falling inside the drop tower is cooled to a supercooled state in the nucleation zone by means of cooling gas, and when the seed crystal generation means generates a seed crystal by adding a stimulus to the particulate melt that is in the supercooled state, the particulate melt is solidified and becomes a spherical crystal. In the lower part of the drop tower, a momentum dissipation zone for dissipating the momentum of the spherical crystal is provided, and this momentum dissipation zone is provided with a curved path which changes the direction of motion of the spherical crystal from the vertical direction to the horizontal direction, and gas flow formation means for reducing the speed, which generates an inert gas flow going from bottom to top.
However, in the spherical crystal producing device of this patent publication, a flow of the cooling gas going toward opposite the falling direction of the melt acts on the falling particulate melt inside the nucleation zone, thus the falling state of the melt becomes a different free falling state. Also an external force acts on the falling particulate melt, thus the structure within the melt easily fluctuates, and the shape of the solidified spherical crystal easily distorts. Thus a single crystal is not necessarily obtained.
The object of the present invention is to provide a drop tube type particulate crystalline body producing device capable of maintaining microgravity created by the free fall of the melt while cooling the particulate melt by means of cooling gas. Another object of the present invention is to provide a drop tube type particulate crystalline body producing device capable of creating a seed crystal by applying an impact to the melt in a supercooled state by means of cooling gas.
A further object of the present invention is to provide a drop tube type particulate crystalline body producing device capable of shortening the height of a drop tube through cooling by means of cooling gas.
Yet another object of the present invention is to provide a drop tube type particulate crystalline body producing device for reducing gas consumption by circulating the cooling gas and for stabilizing the gas pressure by being able to control it.